What made the Merlin engine so great?

Two-stage supercharger on the Merlin.

http://en.wikipedia.org/wiki/Allison_V-1710

 

It was the War Production Board and Army Air Corps the that did not allow Allison two stage two speed supercharger. The only aircraft to get them was the P-38. See here: http://www.aviation-history.com/engines/allison.htm

Interestingly enough, Merlin engines never won Reno air races until someone figured out how to install Allison connecting rods.

 

Actually, the P-38's Allisons were turbocharged, not supercharged, and that made all the difference. The P-38s sent to Britain lacked the turbos and were dogs as a result. To my knowledge, the Merlins were never turbocharged, because they didn't need to be; their superchargers were good enough for strong performance at altitude.

Ironically, the same situation took place after the war in Unlimited hydroplane racing; the Allison ruled the roost until Merlins became plentiful; then the Allison crowd had to put turbochargers on their engines to remain competitive. Today the only piston-powered Unlimited running uses a turbocharged Allison.

 

The Merlin also had an unconvoluted (spl) intake and valves that didn't float. The intake which was more of an open chamber instead of a bunch of manifold pipe-type passages and it made full use of the two stage blower. No Allison has the sound of a Merlin.
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